The viscoelastic properties of a material (such as a biological material), are the resistance to flow (viscosity) and the resistance to deformation (elasticity) of the material. Viscous materials resist shear and strain linearly with time upon application of stress. Elastic materials strain when stretched and return to their original state when the stress is removed. Viscoelastic materials have elements of both of these properties and, as such, exhibit time-dependent strain. The storage and loss modulus (the “complex modulus”) in viscoelastic materials measure the stored energy, representing the elastic portion, and the energy dissipated as heat, representing the viscous portion. Biological materials are rarely just viscous or just elastic, and typically display a combination of viscoelastic properties.
An optical trap includes a focused laser beam able to trap small particles at its focus, and can be used to interrogate the viscoelastic properties of certain materials. However, issues of image resolution and limited depth of interrogation have prevented use of optical trap techniques to measure viscoelastic properties in certain materials, such as biological materials including multi-cellular systems and tissue in living organisms.